MICROSTRUCTURE AND SUPERCONDUCTING PROPERTIES OF NB3AL MULTIFILAMENTARY WIRES PROCESSED BY NB-TUBE METHOD

Microstructure of Nb tube processed Nb3Al multifilamentary wires was studied by an X-ray diffractometer, a transmission electron microscope, etc., and its effect on superconducting properties was discussed from the viewpoint of Al-core sizes, heat treatments and additive elements. Samples with various Al-core diameters were prepared by arranging the filament numbers and final composite diameters. With decreasing Al-core sizes, the volume ratio of A15 phase to other intermediate phase increased, but the lattice parameter of A15 phase decreased conversely. This was accompanied by an increase in T(c) and H(c2) (4.2 K), suggesting that the small diffusion spacing between Nb and Al, which was realized by reducing the Al-core size, facilitates to form the metastable A15 phase with the composition near stoichiometry. However, both the n value of voltage-current characteristics and its magnetic field dependence were remarkably decreased when the Nb/Al composite was heavily cold-drawn into Al-core sizes below 50 nm, suggesting that the Al filament "sausaging" caused by heavy cold-drawing is one of the reasons for degradation in superconducting properties at small Al-core sizes. TEM microstructures were in good agreement with reported ones of Nb/Al multilayer thin films. Alloying Al-cores, which are essential to improve the workability of Nb/Al composites, influences the superconducting properties through the diffusion reaction. Additive elements of Ag and Mg facilitate the formation of the A15 phase with excellent superconducting properties at a low reaction temperature (below 1200 K). They enable the Nb/Al composite to be used for alternating current (ac) superconductors, since ac conductors consisting of resistive Cu alloy matrix must be heat treated below 1200 K. On the other hand, the addition of Cu, which depresses the A15 phase formation rate, is suitable for the high-field superconductors, since it makes it easy to control the high temperature heat treatments for short periods.